1. Field of the Invention
The present invention relates generally to a pulse width modulated signal amplifier, and more particularly is directed to an improved amplifier of the type which converts a low frequency signal, such as, an audio signal or the like, to a pulse width modulated signal, then amplifies the converted signal and demodulates the amplified signal to again obtain the original low frequency signal.
2. Description of the Prior Art
A pulse width modulated signal amplifier is known in which a low frequency input signal, such as, an audio signal or the like, is applied to a have inverter to obtain an inverted input signal which is composed with a rectangular waveform signal from a suitable generator and applied to an integrator. The resulting integrated signal is fed to a high gain amplifier which produces a pulse width modulated signal applied to a pulse power amplifier, and the output signal from the latter is supplied to a low pass filter which delivers an amplified low frequency output signal generally corresponding to the original low frequency input signal. Further, a part of the output of the pulse power amplifier is applied, as a negative feedback, to the input of the integrator.
The above described prior a pulse width modulated signal amplifier is advantageous in that the frequency of the pulse width modulated signal can be made constant and the negative feedback can be sufficient to ensure a desirably low modulation distortion factor. However, there are a number of inherent disadvantages. More particularly, the amplified low frequency output signal obtained from the described pulse width modulated signal amplifier is inverted in phase in respect to the low frequency signal applied to the input of the integrator. Therefore, it is necessary to supply the low frequency input signal to the integrator by way of the phase inverter, as previously described, for ensuring that the input and output signals will be of the same phase. Further, it is necessary to apply the inverted input signal to the integrator through a buffer providing a high input impedance in order to ensure that the gain of the amplifier will not vary with variations in the input signal.